Field of the Invention
This invention relates to separation of HCl from mixtures of the same with fluorine compound contaminants soluble in hydrochloric acid.
In industrial processes, such as the manufacture of chloro-fluorocarbons by fluorination of CCl.sub.4 with HF, reaction gases and vapors are produced which contain relatively large quantities of by-product HCl together with small amounts of flourine compounds, usually in the form of HF and its soluble salts such as SiF.sub.4. Economic considerations make HCl recovery very important. While recovery of the HCl itself raises no particular problems, fluorine contaminants are soluble in HCl and in aqueous solutions thereof, and although the fluorine contaminant content of recovered HCl is relatively low, there is no worthwhile commercial outlet for HCl or hydrochloric acid solutions containing fluorine compounds in amounts normally present in HCl recovered by conventional methods. For example, even in concentrations of 50 ppm and less, hydrofluoric acid can attack glass and enamel. Hence, reduction of the fluorine compound content to acceptably low values presents problems of substantial operating and economic importance. However, due to similarities in properties, it is very difficult to separate HF from HCl by usual procedures.
In patent and other technical literature, various processes have been disclosed which are directed, and with varying degrees of success, to the removal of the contaminant hydrogen fluoride from the by-product HCl in order to render the HCl commercially useful.
Processes which have been developed to remove HF from gases include those in which the gas is contacted with an aqueous solution of alumina (e.g., U.S. Pat. No. 2,920,941, issued in 1960 to J. Sanlaville et al.) and those in which the HF gas is passed over successive solid reaction masses, e.g. silica base and boric acid (U.S. Pat. No. 2,526,777, issued in 1949 to L. B. Smith et al. and solid, substantially anhydrous calcium chloride (German Patent Publication No. 2,229,571, issued in 1972 to D. R. Merchant). However, these processes do not provide sufficiently complete removal of HF, and use of solids to react with HF gives rise to materials handling problems and results in a large waste of solids, due to the formation on the particles of a scale of reaction product (e.g., CaF.sub.2 in the instance of reaction with solid calcium chloride), which prevents reaction of core solid reactant with additional HF. Thus, reaction vessels containing these solids must be regularly serviced to remove inactivated particles and to introduce fresh solids. Equipment utilization during down time is lost to the process, and the repeated venting of the equipment gives rise to environmental pollution problems due to escape to the atmosphere of gases in the reactor.
In the process of U.S. Pat. No. 2,690,815 (issued in 1954 to J. D. Calfee et al.), an aqueous HF solution is employed to preferentially absorb HF from HCl/HF gas mixtures, and in the methods of U.S. Pat. No. 2,345,696 (issued in 1944 to A. F. Benning et al.) and Canadian Pat. No. 835,853 (issued in 1970 to P. Frisch et al.), an HCl/HF gas mixture is contacted with a concentrated HCl solution to absorb HF. However, the HCl gas thereby produced, when recovered by adiabatic absorption in water, provides an aqueous hydrochloric acid which is unsuitable for many purposes, since the exit HCl gas still contains small amounts of HF. Moreover, such processes require the handling of large volumes of highly corrosive mixtures of hydrofluoric and hydrochloric acids.
To obtain more complete HF removal while still retaining the process advantages of a liquid absorption process, the prior art has developed processes in which dissolved HF is caused to react with an added reactant to precipitate CaF.sub.2. Thus, U.S. Pat. No. 3,743,704 (issued in 1973 to N. E. West) relate to a process in which HF gas is absorbed in water and Ca(OH).sub.2, CaCO.sub.3 or a mixture thereof added to the aqueous HF solution to precipitate CaF.sub.2. In Dutch Patent Publication No. 7,003,378 (laid open to public inspection on Sept. 15, 1970) HF gas is absorbed using an aqueous Ca(OH).sub.2 solution to precipitate CaF.sub.2 directly. However, these processes require the addition to the settling zone of a second substance, e.g. soluble alkaline earth salts such as CaCl.sub.2 and MgCl.sub.2, to allow effective settling of the fine CaF.sub.2 particles, which would otherwise form a slow-settling gelatinous precipitate. In addition, these processes require careful pH control to provide a CaF.sub.2 precipitate which may be easily removed.
In the process of U.S. Pat. No. 2,507,603 (issued in 1950 to A. J. Killgore), escaping gaseous HF catalyst is neutralized by absorption with an aqueous CaCl.sub.2 solution to form a solution containing CaF.sub.2 solids, which is then contacted with a carbonate, such as CaCO.sub.3, to regenerate the CaCl.sub.2 solution for recycle to the HF absorption step. However, such a process is not readily adaptable to separation of HF from HCl/HF gas mixtures in which recovery of HCl is also desired, and the product results in HCl with unacceptably high residual content of HF. Thus, this patent describes an exemplary process (at column 4, line 51 to column 5, line 32) in which the "substantially hydrogen-free" HCl which was recovered contained 0.02 to 0.3% HF.
U.S. Pat. No. 3,140,916 (issued in 1964 to Lowdermilk) disclosed a process wherein product gases from the production of chloro-fluorohydrocarbons are initially scrubbed with a dilute solution of hydrochloric acid to absorb substantially all of the HF and HCl from the product gases. The solution containing HF and HCl is then continuously fed to a heated stripping tower which produces anhydrous hydrogen chloride as overhead and a bottoms product containing HCl and HF. After the bottoms product builds up to in excess of 1% HF, it is cooled and treated with an aqueous CaCl.sub.2 solution to form CaF.sub.2 crystals which are removed from the parent liquor by filtration. While the foregoing process successfully removes a portion of the hydrogen fluoride from the by-product HCl/HF gas stream, the process has a serious deficiency in that the HCl gas produced still contains substantial quantities of HF. Moreover, the reference requires use of CaCl.sub.2 in less than stoichiometric amounts to ensure at least about 1 wt.% HF remains in the bottoms, thereby increasing the HF vapor pressure of recycle to the stripper and, consequently, the HF in the HCl overhead. More complete conversion of HF to calcium fluoride and, hence, lower residual HF concentrations, are not obtained since greater amounts of a gelatinous deposit of calcium chloride were formed thereby leading to severe processing difficulties.
In the process of Canadian Pat. No. 556,772 (issued in 1958 to L. Foulletier et al.) an HCl/HF gas is contacted with a saturated aqueous HCl solution containing dissolved metallic salt such as calcium or aluminum chloride. The HF which is absorbed reacts to form a metallic fluoride precipitate. However, this process does not cure the problems associated with removal of the CaF.sub.2 precipitate (which is formed using CaCl.sub.2) and is also disadvantaged by the residual HF content of the HCl gas exiting the process (e.g., 0.03 molecule % HF using aluminum chloride in Example 3 at column 4, line 72) which is still too high for many uses of HCl.
Accordingly, there is a need for a process which would provide efficient removal of greater quantities of hydrogen fluoride from by-product hydrogen chloride gaseous streams.